When data are transmitted without the use of error protection schemes such as parity bits or error correcting codes it is not possible to tell at a receiver whether any particular bit has been correctly received. If such schemes are used, error burst detection is possible but fails if the incidence of errors exceeds that which the scheme can cope with. In both cases, it is self-evident that detection of individual bit errors is not possible. However, in many types of channel the errors are correlated--i.e. errors tend to occur in bursts--and the aim of the present invention is to identify periods of reception during which there is high probability of errors having occurred; in other words, to estimate the occurrence and duration of bursts.